Two stage transcritical refrigeration system

ABSTRACT

The invention provides a refrigeration system including a plurality of refrigerated display cases. Each display case has a dedicated evaporator assembly that cools return air by evaporating a refrigerant, and a first stage compressor assembly that is in fluid communication with the respective evaporator assembly to compress the refrigerant to a first pressure corresponding to a first temperature of the refrigerant prior to discharge of the compressed refrigerant. The refrigeration system also includes a second stage compressor assembly that is in fluid communication with the first stage compressor assemblies to compress the refrigerant to a second pressure that is higher than the first pressure, and that corresponds to a second temperature of the refrigerant. The refrigeration system further includes a heat exchanger that is in fluid communication with the second stage compressor assembly to receive the refrigerant from the second stage compressor assembly to reject heat from the refrigerant.

RELATED APPLICATIONS

This patent application claims priority to U.S. Patent Application Ser. No. 60/858,624, filed Nov. 13, 2006, the entire contents of which are hereby incorporated by reference.

BACKGROUND

The present invention relates to a refrigeration system, and more specifically, to a transcritical refrigeration system for refrigerating food product displayed in refrigerated display cases in a commercial application.

A retail store, such as a supermarket, typically contains many refrigerated display cases for displaying and cooling food and/or beverage items for sale. Many types of refrigerated display cases are known in the art, and are in extensive use in retail locations. Such refrigerated display cases require a refrigeration system to maintain a temperature within the display case that is lower than ambient temperature inside the store.

Refrigeration systems generally include an evaporator, a compressor, and a condenser. A refrigerant fluid flows from one component to the next, exchanging heat so as to absorb heat from a refrigerated area and reject heat at the condenser, typically experiencing a phase change during the cycle.

One example of a prior art refrigeration system 10 is shown in FIG. 1. Each refrigerated display case 15 includes an evaporator 20 for removing heat from each refrigerated display case 15. Evaporated refrigerant is routed from the evaporators 20 via a suction header 25 to a local bank of compressors 30 and then through a discharge header 35 to a remotely located condenser 40 to be condensed. Condensed refrigerant is routed from the condenser 40 via a liquid header 45 to the evaporators 20. The local bank of compressors 30 is located either at the end of a group or directly atop a group of refrigerated display cases 15 and contains several compressors connected in parallel within a sound-attenuated casing 50. The suction header 25 and the discharge header 35 are partially located within the sound-attenuated casing 50. The discharge header 35 establishes fluid communication between the local bank of compressors 30 and the condenser 40 and is not necessarily positioned adjacent each refrigerated display case 15. Similarly, the liquid header 45 establishes fluid communication between the evaporators 20 and the condenser 40, and is not necessarily positioned adjacent each refrigerated display case 15. The local bank of compressors 30 serves to compress heated refrigerant from several evaporators 20. The remotely located condenser 40 receives heated refrigerant from the single local bank of compressors 30.

SUMMARY

In one embodiment, the invention provides a refrigeration system for use in a retail store application that includes a plurality of refrigerated display cases. Each of the plurality of refrigerated display cases has a dedicated evaporator assembly that is adapted to cool return air from the respective refrigerated display case by at least partially evaporating a refrigerant. Each refrigerated display case also has a dedicated first stage compressor assembly that is in fluid communication with the dedicated evaporator assembly to compress the refrigerant from the dedicated evaporator assembly to a first pressure that corresponds to a first temperature of the refrigerant, and to discharge the compressed refrigerant into a discharge main.

The refrigeration system also includes a second stage compressor assembly and a heat exchanger. The second stage compressor assembly is in fluid communication with the dedicated first stage compressor assembly of each of the plurality of refrigerated display cases to receive the refrigerant and to compress the refrigerant to a second pressure that is higher than the first pressure, and that corresponds to a second temperature of the refrigerant. The heat exchanger is located remotely from the plurality of refrigerated display cases, and is in communication with the refrigerated display cases via a fluid main. The heat exchanger includes an inlet that is in fluid communication with the second stage compressor assembly to receive the refrigerant from the second stage compressor assembly to reject heat from the refrigerant to an environment. The heat exchanger also includes an outlet that is in fluid communication with the dedicated evaporator assembly in each of the plurality of refrigerated display cases via the fluid main.

In another embodiment, the invention provides a refrigeration system that includes a low temperature refrigeration circuit circulates a first refrigerant, and a high temperature refrigeration circuit circulates a second refrigerant. The low temperature refrigeration circuit includes a plurality of refrigerated display cases, each having a dedicated evaporator assembly that is adapted to cool return air from the respective refrigerated display case by at least partially evaporating the first refrigerant. Each display case also includes a dedicated first stage compressor assembly that is in fluid communication with the dedicated evaporator assembly to compress the first refrigerant from the dedicated evaporator assembly to a first pressure that corresponds to a first temperature of the first refrigerant.

The high temperature refrigeration circuit includes a second compressor assembly that is operable to compress the second refrigerant to a second pressure that corresponds to a second temperature of the second refrigerant. The high temperature refrigeration system also includes a heat exchanger that cools the second refrigerant. The refrigeration system also includes a heat exchanger that is in communication with the low temperature refrigeration circuit and the high temperature refrigeration circuit to transfer heat from the first refrigerant to the second refrigerant to cool the first refrigerant without mixing the first refrigerant and the second refrigerant.

In yet another embodiment, the invention provides a refrigeration system that includes a plurality of refrigerated display cases, each having a dedicated evaporator assembly adapted to cool return air from the respective refrigerated display case by at least partially evaporating a refrigerant. The refrigeration system also includes at least one first stage compressor assembly, a second stage compressor assembly, and a heat exchanger. The first stage compressor assembly is in fluid communication with the dedicated evaporator assembly to compress the refrigerant from the dedicated evaporator assembly to a first pressure that corresponds to a first temperature of the refrigerant.

The second stage compressor assembly is in fluid communication with the at least one first stage compressor assembly to receive the compressed refrigerant and to compress the refrigerant to a second pressure that is higher than the first pressure, and that corresponds to a second temperature of the refrigerant. The heat exchanger is located remotely from the plurality of refrigerated display cases, and includes an inlet that is in fluid communication with the second stage compressor assembly to receive the refrigerant from the second stage compressor assembly and to reject heat from the refrigerant to an environment. The refrigeration system also includes a vessel that is positioned between and in fluid communication with the dedicated evaporator assemblies and the heat exchanger to separate liquid and gaseous phases of the refrigerant. The refrigeration system further includes a liquid main and a gas main. The liquid main is fluidly coupled to the dedicated evaporator assemblies and to the vessel, and supplies liquid refrigerant to each dedicated evaporator assembly. The gas main is fluidly coupled to the second stage compressor assembly and to the vessel, and directs gaseous carbon dioxide refrigerant from the vessel to the second stage compressor assembly without passing through the dedicated evaporator assemblies.

In yet another embodiment, the invention provides a refrigeration system that includes a closed coolant loop that circulates a coolant fluid, and that includes a supply main, a distribution main, and a discharge main. The coolant loop also includes a heat exchanger that is fluidly connected to the discharge main and the supply main and in communication with an environment to reject heat from the coolant fluid into the environment, and at least one coolant pump fluidly connected to the distribution main and the discharge main to pump the coolant fluid to the heat exchanger and through the coolant loop. The refrigeration system also includes a plurality of refrigerated display cases that are coupled to the coolant loop. Each of the plurality of refrigerated display cases includes a dedicated evaporator assembly that cools return air from the respective refrigerated display case by at least partially evaporating a refrigerant, and a dedicated refrigeration unit that is coupled to the evaporator assembly.

The evaporator assembly and the refrigeration unit are in fluid communication with each other, and define a closed transcritical refrigeration circuit. The refrigeration unit includes a compressor assembly that is in fluid communication with the evaporator assembly to compress the refrigerant from the evaporator assembly, and a heat exchanger in fluid communication with the compressor assembly and with the evaporator assembly. The heat exchanger is in heat exchange relationship with the coolant loop to reject heat from the refrigerant to the coolant fluid.

Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a prior art refrigeration system for refrigerating display cases.

FIG. 2A is a schematic view of a transcritical refrigeration system for refrigerating display cases.

FIG. 2B is a schematic view of another embodiment of a transcritical refrigeration system that includes a cascade refrigeration circuit for refrigerating display cases.

FIG. 3 is a schematic view of another embodiment of a transcritical refrigeration system that includes a closed coolant loop and a transcritical refrigeration unit for refrigerating display cases.

FIG. 4 is a schematic view of one refrigerated display case of FIG. 3 that includes a transcritical refrigeration unit of FIG. 3.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings.

FIG. 2A shows a refrigeration system 100 for use with a building 105 that includes a shopping area 110. As used herein, the term “shopping area 110” refers to the commonly accessible area of a supermarket where customers may browse items for sale, and generally does not include any areas designated as equipment, storage, or maintenance areas.

The refrigeration system 100 is a transcritical refrigeration system that circulates carbon dioxide refrigerant (“CO₂ refrigerant”) as a cooling fluid. The refrigeration system 100 includes refrigerated display cases 115, a first stage compressor assembly 120 attached to each of the display cases 115, a second stage compressor assembly 125, a gas cooler or heat exchanger 130, a vessel 135, a fluid main 140, and a discharge main 145. The refrigerated display cases 115 are positioned throughout the shopping area 110 of the building 105 for housing and displaying food product.

Each refrigerated display case 115 is fluidly connected to the fluid main 140 via a liquid branch line 150. Each liquid branch line 150 is fluidly connected to the fluid main 140 in parallel with each of the remaining liquid branch lines 150. The refrigerated display cases 115 are further fluidly coupled to the discharge main 145 via discharge branch lines 155. Each discharge branch line 155 is fluidly coupled to the discharge main 145 in parallel with each of the remaining discharge branch lines 155.

Each of the refrigerated display cases 115 includes an expansion device or valve 160, and an evaporator assembly 165 coupled to the related first stage compressor assembly 120. For illustrative purposes, only the expansion device 160 and the evaporator assembly 165 of one of the display cases 115 are shown. The expansion device 160 is located between the fluid main 140 and the evaporator assembly 165 to regulate a pressure of refrigerant flowing from the fluid main 140 to the evaporator assembly 165.

The illustrated evaporator assembly 165 of each display case 115 includes a single dedicated evaporator coupled to the related first stage compressor assembly 120. In other embodiments, the evaporator assembly 165 may employ more than one evaporator, with each of the evaporators connected in parallel or series between the liquid branch line 150 and the first stage compressor assembly 120.

As used herein, the phrase “evaporator assembly 165” does not imply the use of any particular refrigerant (i.e., a two-phase refrigerant or a single-phase refrigerant). Rather, the terms should be generally construed to describe a heat exchanger assembly/coil functioning to transfer heat from an airflow passing through/over the heat exchanger assembly/coil to a refrigerant flowing through the heat exchanger assembly/coil.

Each first stage compressor assembly 120 is coupled to the discharge branch line 155 between one of the display cases 115 and the discharge main 145. The illustrated first stage compressor assembly 120 includes a single dedicated compressor to compress refrigerant received from the evaporator assembly 165 of the related display case 115. Other embodiments of the first stage compressor assembly 120 may include multiple dedicated compressors for a single display case 115. Still other embodiments may include one or more compressors for a small group of display cases 115.

The second stage compressor assembly 125 is located downstream of the first stage compressor assembly 120. The illustrated embodiment of the second stage compressor assembly 125 shows the second stage compressor assembly 120 including two compressors 170. In other embodiments, the second stage compressor assembly 120 may include one compressor 170, or more than two compressors 170. In some embodiments, the compressors 170 can be one or more variable capacity compressors. In these embodiments, the capacity of the compressors can be varied to accommodate pressure fluctuations that may be present within the refrigeration system 100.

The second stage compressor assembly 125 is in fluid communication with the first stage compressor assembly 120 via a fluid line 172. The fluid line 172 includes an inlet 173 coupled to an upper portion of the vessel 135, and extends between the inlet 173 and an inlet line 175 of the second stage compressor assembly 125. The second stage compressor assembly 125 further includes an outlet line 180 that extends between an outlet of the compressor assembly 125 and the heat exchanger 130.

A bypass line 182 is coupled between the fluid line 172 and the outlet line 180 to circulate gaseous CO₂ refrigerant from the fluid line 172 to the heat exchanger 130 without passing through the second stage compressor assembly 125. A check valve 183 is positioned in the bypass line 182 to allow refrigerant flow toward the heat exchanger 130 and to limit refrigerant flow from the outlet line 180 backward to the fluid line 172.

The heat exchanger 130 is located remotely from the shopping area 110, and further outside the building 105 to cool the CO₂ refrigerant. In some embodiments, the heat exchanger 130 may be located inside the building 105 but remote from the shopping area 110. The heat exchanger 130 includes an inlet 185, an outlet 190, and at least one fan 195 to cool the CO₂ refrigerant. The inlet 185 is in fluid communication with the second stage compressor assembly 125 via the gas main 175, and the outlet 190 is in fluid communication with the vessel 135.

The vessel 135 is in fluid communication with the fluid main 140 to separate liquid CO₂ refrigerant from gaseous CO₂ refrigerant, and can be located anywhere along the fluid main 140 between the heat exchanger 130 and the refrigerated display cases 115 (i.e., either inside the building 105 or outside the building 105), without departing from the scope of the invention. A bypass main 197 is coupled between the vessel 135 and the fluid line 172 to circulate CO₂ refrigerant from the vessel 135 to the second stage compressor assembly 125 without passing through the fluid main 140 to the display cases 115. An expansion device 198 is disposed along the fluid main 140 downstream of the heat exchanger 130 and upstream of the vessel 135 to regulate the pressure of the CO₂ refrigerant exiting the heat exchanger 130.

In the illustrated embodiment, the fluid main 140 and the discharge main 145 are routed throughout the building 105, such that at least a portion of the fluid main 140 and at least a portion of the discharge main 145 are positioned adjacent each refrigerated display case 115 of the refrigeration system 100. A downstream end of the discharge main 145 is in heat exchange relationship with the vessel 135, and in fluid communication with the inlet 173. The heat exchange between the superheated refrigerant in the discharge main 145 and the refrigerant in the vessel 135 adequately cools the superheated refrigerant prior to entry into the second stage compressor assembly 125. In other embodiments, the downstream end of the discharge main 145 may be in fluid communication with the vessel 135. In those embodiments, the refrigerant from the discharge main 145 can be mixed with the refrigerant prior to being directed to the second stage compressor assembly 125.

Positioning at least a portion of both the fluid main 140 and the discharge main 145 adjacent each refrigerated display case 115 allows the refrigerated display cases 115 to be installed at a variety of locations in the shopping area 110 by tapping into the fluid main 140 and the discharge main 145 with the pair of respective liquid and discharge branch lines 150,155. In some embodiments, the fluid and discharge mains 140,145 may be outside the shopping area 110 adjacent a display case that is near an edge (e.g., a wall) of the shopping area 110. In other embodiments, the fluid and discharge mains 140, 145 may extend out into the shopping area 110 adjacent each of a group of more centrally located display cases 115.

In still other embodiments, the store may be divided into “sub-loop” areas with a fluid and discharge main 140, 145 and an air-cooled heat exchanger 130 for each sub-area of the store. For example one set of fluid and discharge mains 140, 145 with the air-cooled heat exchanger 130 may be positioned on a left side of the store, and a separate set of fluid and discharge mains 140, 145 and a separate air-cooled heat exchanger 130 may be positioned on a right side of the store. A further embodiment may use such separate sub-loops for different types of refrigerated display cases 115. For example, one set of fluid and discharge mains 140, 145 for the meat area, one set for the produce area, etc. Thus, the fluid main 140 and the discharge main 145 allow the refrigerated display cases 115 to be positioned throughout the shopping area 110 of the building 105 in a variety of configurations, without requiring extensive routing of lengthy individual liquid and discharge branch lines 150, 155 and thereby minimizing the length of the liquid and discharge branch lines 150, 155.

The arrangement of the fluid main 140 and discharge main 145 throughout the building 105 may simplify the installation procedure of refrigerated display cases 115 or allow for the refrigerated display cases 115 to be easily moved from one location to another within the building 105. The fluid and discharge mains 140, 145 may be plumbed into the building 105 (e.g., under the floor or behind walls of the shopping area 110) before any refrigerated display cases 115 are installed. When refrigerated display cases 115 are ready to be installed, they can be added to the refrigeration system 100 by tapping into the fluid and discharge mains 140, 145 at a location very near the desired location for the refrigerated display case 115. This eliminates the need for routing lengthy liquid or discharge branch lines 150, 155 between each of the refrigerated display cases 115 and a centralized location, remote from the shopping area 110. This also improves the modularity of the shopping area 110, in that the refrigerated display cases 115 can be reconfigured and moved throughout the shopping area 110 without requiring the cumbersome activity of re-routing lengthy liquid and discharge branch lines 150, 155.

The transcritical CO₂ refrigeration cycle of the refrigeration system 100 operates in a similar fashion to the reverse-Rankine refrigeration cycle, except the vapor CO₂ refrigerant is compressed to a temperature above the thermodynamic critical point of the CO₂ refrigerant (i.e., approximately 87.7 degrees Fahrenheit). As described in detail below, the CO₂ refrigerant is compressed to a high, transcritical pressure of about 1600 psig corresponding to a temperature above the critical point by the second stage compressor assembly 125 prior to cooling of the refrigerant in the heat exchanger 130. Consequently, when heat is rejected from the CO₂ refrigerant in the heat exchanger 130, the vapor CO₂ refrigerant is cooled to a cooled vapor rather than changing phases to a liquid. In the closed circuit travel of the CO₂ refrigerant, the heat exchanger 130 cools the high-pressure CO₂ vapor to a lower temperature as a result of the forced airflow generated by the fan 195. In the illustrated embodiment, the heat exchanger 130 cools the CO₂ refrigerant from about 250 degrees Fahrenheit to about 100 degrees Fahrenheit. In other embodiments, the temperature of the CO₂ refrigerant may be cooled by the heat exchanger 130 from temperatures above or below 250 degrees Fahrenheit to temperatures above or below 100 degrees Fahrenheit.

The cooled, high-pressure vapor CO₂ refrigerant is then throttled through the expansion device 198 to an intermediate, subcritical pressure fluid of about 600 psig where, similar to the reverse-Rankine refrigeration cycle, the intermediate-pressure CO₂ refrigerant changes phase to a liquid-vapor mixture prior to entering the vessel 135. In the illustrated embodiment, the difference between the high pressure of the refrigerant and the intermediate pressure of the refrigerant is approximately 1000 psig. In other embodiments, the difference between the high pressure of the refrigerant and the intermediate pressure of the refrigerant can be above or below 1000 psig.

The liquid-vapor mixture then enters the vessel 135 where the liquid CO₂ refrigerant is separated from gaseous CO₂ refrigerant. The gaseous CO₂ refrigerant can flow through the bypass main 197 to the inlet 175 of the second stage compressor assembly 125, while the liquid CO₂ refrigerant can flow through the remaining portion of the fluid main 140 to the refrigerated display cases 115 via the liquid branch lines 150.

The intermediate pressure CO₂ refrigerant is throttled by the expansion device 160 of each display case 115 to a low-pressure refrigerant prior to entry into the evaporator assembly 165. The low-pressure refrigerant boils to a vapor in the evaporator assembly 165. In other words, the low-pressure CO₂ refrigerant passing through the evaporator assembly 165 absorbs the heat from the airflow as it is passed through the evaporator assembly 165, thereby cooling the airflow.

The gaseous CO₂ refrigerant flows from the evaporator assembly 165 to the first stage compressor assembly 120 where the refrigerant is compressed to the intermediate, subcritical pressure that corresponds to a temperature generally below the critical point. While the intermediate pressure of the refrigerant in the illustrated embodiment is at about 600 psig, other pressures of the refrigerant higher and lower than 600 psig are possible.

The intermediate pressure refrigerant flows from the first stage compressor assembly 120 through the discharge branch lines 155, and accumulates in the discharge main 145. The refrigerant flows through the discharge main 145 to the vessel 135, where the refrigerant can be de-superheated. The refrigerant is de-superheated prior to entering the second stage compressor assembly 125 by heat exchange with the refrigerant in the vessel 135. The de-superheated refrigerant flows from the discharge main 145 into the fluid line 172 toward the inlet 175. The second stage compressor assembly 125 receives the intermediate pressure refrigerant from one or both of the discharge main 145 and the bypass main 197 through the inlet 175. The refrigerant flowing from the bypass main 197 from the vessel 135 is at about the same intermediate pressure as the refrigerant flowing from the fluid line 172.

The second stage compressor assembly compresses the CO₂ refrigerant from the intermediate pressure to the high pressure. While the high pressure refrigerant in the illustrated embodiment is at about 1600 psig, other pressures of the refrigerant higher and lower than 1600 psig are capable using the second stage compressor assembly 125, and are within the scope of the invention. The high pressure refrigerant flows from the second stage compressor assembly 125 through the outlet line 180, and is cooled in the heat exchanger 130 as described above.

To obtain desirable refrigeration characteristics from the CO₂ refrigerant, the transcritical CO₂ refrigeration cycle requires higher operating pressures compared to a reverse-Rankine refrigeration cycle using R134a, for example. In some applications, the pressure experienced in the heat exchanger 130 in the transcritical CO₂ refrigeration system 100 can exceed the pressure experienced in a condenser of a reverse-Rankine refrigeration cycle using R134a by as much as eight-fold. Also, the low pressure experienced in the evaporator assembly 165 in the transcritical CO₂ refrigeration cycle can exceed the pressure experienced in an evaporator assembly in a reverse-Rankine refrigeration cycle using R134a by as much as fifteen-fold. As a result, the heat exchanger 130 and evaporator assembly 165 employ a heavy-duty construction to withstand the increased pressure of the transcritical CO₂ refrigeration cycle. Such heavy-duty construction may comprise an increased thickness of the walls of the tubing in the evaporator assembly 165 and heat exchanger 130. In addition, the thickness of the walls of the fluid main 140, the discharge main 145, the liquid branch line 150, the discharge branch line 155, and other conduit (not shown) utilized in the refrigeration system 100 to fluidly connect the refrigeration components may also be increased to accommodate the increased pressure of the transcritical CO₂ refrigeration cycle.

The critical temperature of CO₂ refrigerant is approximately 88 degrees Fahrenheit. In some embodiments of the refrigeration system 100, the ambient temperature surrounding the heat exchanger 130 may drop below the critical temperature of the refrigerant. In those embodiments, the heat exchanger 130 may function similarly to a condenser, cooling the high pressure CO₂ refrigerant to a liquid-vapor or a liquid-only refrigerant. More specifically, variations in the pressure of the refrigerant within the refrigeration system 100 are controlled by changing the state of the second stage compressor assembly 125. For example, when the refrigerant pressure in the discharge main 145 drops below a predetermined pressure that corresponds to a temperature at or below the critical point of the refrigerant, the second stage compressor assembly 125 can be shutdown such that the refrigerant can be cycled through the system 100 using only the first stage compressor assembly 120. When the second stage compressor assembly 125 is shutdown, the refrigerant bypasses the second stage compressor assembly 125 through the bypass line 182, and flows into the heat exchanger 130. When the refrigerant pressure in the discharge main 145 increases above the predetermined pressure, the second stage compressor assembly can be re-activated to accommodate the increased pressure within the refrigeration system 100.

The second stage compressor assembly 125 capacity can be reduced or eliminated as the ambient temperature decreases below the critical temperature to accommodate a decrease in refrigerant pressure within the refrigeration system 100. In embodiments where the ambient temperature is significantly below the critical temperature of the refrigerant, the second stage compressor assembly 125 may be shutdown due to a low refrigerant pressure in the discharge main 145 that corresponds to the temperature below the critical temperature. In these embodiments, the first stage compressor assembly 120 of each display case 115 can operate the entire system 100, entirely bypassing the second stage compressor assembly 125 through the bypass line 182. The check valve 183 prevents backward flow of refrigerant from the outlet line 180 to the fluid line 172 when the second stage compressor assembly 125 is running.

The use of the first and second stage compressor assemblies 120, 125 allows smaller compressors to be positioned adjacent the display cases 115, which limits parasitic losses that may otherwise occur when the compressor assembly 120 is located remotely from the display case 115. The losses are limited because the position of the first stage compressor assemblies 120 adjacent to the display cases 115 allows the first stage compressor assemblies 120 to operate at a desired suction pressure, which maintains an efficient refrigeration system 100. In addition, the close position of the first stage compressor assemblies 120 relative to the display cases 115 allows modulation of the capacity and/or the evaporator temperature of each display case 115 and improves the efficiency of the refrigeration system 100.

FIG. 2B shows another embodiment of a refrigeration system 200 for use with the building 105. Except as described below, the refrigeration system 200 is the same as the refrigeration system 100, and common elements have been given the same reference numerals. The refrigeration system 200 is a split-stage transcritical refrigeration system that includes a high temperature refrigeration circuit 205 and a low temperature refrigeration circuit 210.

The high temperature refrigeration circuit 205 includes the heat exchanger 130, a high temperature receiver 215, a cascade cooler or heat exchanger 220, an accumulator 225, and a compressor assembly 230. The receiver 215 is fluidly connected to the heat exchanger 130 to receive cooled refrigerant. When the high temperature refrigeration circuit 205 is operating above the critical point for the refrigerant, only gaseous refrigerant is stored in the receiver 215. When the high temperature refrigeration circuit 205 is operating at or below the critical point for the refrigerant, some liquid refrigerant may be present in the receiver 215. The expansion device 198 is positioned downstream of the receiver 215 to control the refrigerant discharge pressure. Refrigerant passing through the expansion device 198 flows to the cascade cooler 220 via a fluid line 235.

The high temperature refrigeration circuit 205 is in heat exchange relationship with the low temperature refrigeration circuit 210 via the cascade cooler 220. The cascade cooler 220 includes a first inlet 222 that is fluidly connected to the fluid line 235 to receive cooled refrigerant from the heat exchanger 130. The cascade cooler 220 further includes a first outlet 224 that is fluidly connected to the accumulator 225 via a fluid main 240 to deliver heated refrigerant to the accumulator 225.

The refrigerant entering the cascade cooler 220 from the refrigeration circuit 205 is cooler than the refrigerant entering the cascade cooler 220 from the refrigeration circuit 210. As such, the cascade cooler 220 functions similar to an evaporator for the refrigeration circuit 205, and functions similar to a condenser for the refrigeration circuit 210. In other words, heat from the refrigerant in the refrigeration circuit 210 is absorbed by the refrigerant in the refrigeration circuit 205, thereby cooling the refrigerant in the refrigeration circuit 210 and heating the refrigerant in the refrigeration circuit 205.

The compressor assembly 230 is located downstream of the accumulator 225, and is dedicated to the refrigeration circuit 205. The illustrated embodiment shows the second stage compressor assembly 230 including two compressors 245. In other embodiments, the first compressor assembly 230 may include one compressor 245, or more than two compressors 245. The inlet line 175 fluidly connects the accumulator 225 to the compressor assembly 230.

The low temperature refrigeration circuit 210 operates at a refrigerant temperature that is at or below the critical point for the refrigerant. The refrigeration circuit 210 includes the refrigerated display cases 115, a receiver 250, and compressor assemblies 255. As discussed above with regard to FIG. 2A, each display case 115 is fluidly connected to the fluid main 140 via the liquid branch line 150. The fluid main 140 is coupled to the receiver 250 to distribute cooled refrigerant to the display cases 115.

The refrigerated display cases 115 are further fluidly connected to the discharge main 145 via the discharge branch lines 155. The discharge main 145 is fluidly connected to a second inlet 260 of the cascade cooler 220 to deliver heated refrigerant from the compressor assemblies 255 to the cascade cooler 220.

The receiver 250 is fluidly connected to the cascade cooler 220 via a distribution line 265. The distribution line 265 is coupled to a second outlet 270 of the cascade cooler 220 to direct cooled refrigerant into the receiver 250. The receiver 250 is further in fluid communication with the fluid main 140 to separate liquid refrigerant from gaseous refrigerant, and can be located anywhere along the fluid main 140 upstream of the display cases 115.

Each compressor assembly 255 is dedicated to the refrigeration circuit 205, and is attached to one of the cases 115. Each compressor assembly 255 is further coupled to the discharge branch line 155 between one of the display cases 115 and the discharge main 145. In some embodiments, each compressor assembly 255 may be located remotely from each of the display case 115, with adequate capacity to compress refrigerant from each display case 115. The illustrated compressor assembly 255 includes a single dedicated compressor to compress refrigerant received from the evaporator assembly 165 of the related display case 115. Other embodiments of the compressor assembly 255 may include multiple dedicated compressors for a single display case 115. Still other embodiments may include one or more compressors for a small group of display cases 115.

The transcritical CO₂ refrigeration cycle of the refrigeration system 200 provides cooling to the display cases 115 without mixing refrigerant between the high temperature refrigeration circuit 205 and the low temperature refrigeration circuit 210. Refrigerant entering the cascade cooler 220 through the first inlet 222 is heated by heat exchange with refrigerant flowing through the low temperature refrigeration circuit 210. The heated refrigerant exits the cascade cooler 220 through the first outlet 224 and flows through the fluid main 240 to the accumulator 225. When the refrigeration circuit 205 is operating at or below the critical point for the refrigerant, some liquid refrigerant exists in the accumulator 225. Gaseous refrigerant is separated from the liquid refrigerant in the accumulator 225, and the gaseous refrigerant is then directed to the compressor assembly 230. The gaseous, heated refrigerant is then compressed by the compressor assembly 230 prior to being cooled in the heat exchanger 130. The cooled refrigerant flows from the heat exchanger 130 to the vessel 135, where the high temperature refrigeration circuit 205 begins anew.

The heat exchange relationship between the high temperature refrigeration circuit 205 and the low temperature refrigeration circuit 210 in the cascade cooler 220 cools previously heated refrigerant in the refrigeration circuit 210, and heats previously cooled refrigerant in the high temperature refrigeration circuit 205. The cooled refrigerant in the low temperature refrigeration circuit 210 flows from the second outlet 270 of the cascade cooler 220 and into the receiver 250 via the distribution line 265. The cooled refrigerant flows to each of the display cases 115, where the refrigerant is heated as it passes through the evaporators 165. The heated refrigerant from each display case 115 is compressed by the respective compressor assembly 255 prior to reaching the discharge main 145. The heated refrigerant flows from the compressor assembly 255 to the cascade cooler 220 through the second inlet 260, where the low temperature refrigeration circuit 210 begins anew.

FIG. 3 shows yet another refrigeration system 300 for use with the building 105. The refrigeration system 300 includes a plurality of refrigerated display cases 315 that are coupled to a coolant loop 317. Each refrigerated display case 315 includes an evaporator assembly 320 and a refrigeration unit 325 that is coupled to the evaporator assembly 320 (FIG. 4). The evaporator assembly 320 is located such that air passing through the evaporator assembly 320 is discharged to a refrigerated area 330 of the refrigerated display case 315. The evaporator assembly 320 and the refrigeration unit 325 are each dedicated to operate with only one of the refrigerated display cases 315. As shown in FIG. 3, the evaporator assembly 320 includes one evaporator 335 to provide cooling to the refrigerated area 330. However, the quantity of evaporators 335 depends on the cooling requirements of each refrigerated display case 315, and additional evaporators 335 may be included in the evaporator assembly 320 without deviating from the scope of the invention.

FIG. 4 illustrates the refrigeration unit 325 of one refrigerated display case 315. Each refrigeration unit 325 defines a transcritical refrigeration cycle that circulates CO₂ refrigerant to cool the refrigerated area 330. The refrigeration unit 325 includes a compressor assembly 340 and a gas cooler or heat exchanger 345. In some embodiments, each refrigeration unit 325 may further include a receiver (not shown) coupled to the compressor assembly 340.

The compressor assembly 340 is coupled to the evaporator assembly 320 to compress CO2 refrigerant received from the evaporator assembly 320. The illustrated compressor assembly 340 includes two dedicated compressors 350 that are placed within the transcritical refrigeration cycle in parallel with each other. Other embodiments of the compressor assembly 340 may include one compressor, or more than two compressors 350 in parallel or in series with each other.

The heat exchanger 345 includes an inlet 355 that is coupled to the compressor assembly 340, and an outlet 360 that is coupled to the evaporator assembly 320. As shown in FIG. 3, the heat exchanger 345 is in heat-exchange relationship with the coolant loop 317 to reject heat from the compressed CO₂ refrigerant to a coolant fluid in the coolant loop 317. In some embodiments, an expansion device (not shown) can be located adjacent the outlet 360 to expand the CO₂ refrigerant prior to reaching the evaporator assembly 320.

The CO₂ refrigerant in the heat exchanger 345 is cooled by heat exchange with the coolant fluid that is circulated in the coolant loop 317. In one embodiment, the coolant fluid is a water/glycol mixture. As shown in FIG. 3, the coolant loop 317 is a closed circulation coolant loop, and includes an air-cooled heat exchanger 365, a supply main 370, a distribution main 375, coolant pumps 380, and a discharge main 385. The heat exchanger 365 is located remotely from the shopping area 110, and is in communication with an environment surrounding the building 105 to cool the coolant fluid. The heat exchanger 365 includes a fan 390 to draw air across the heat exchanger 365 to cool the coolant fluid. In some embodiments, the heat exchanger 365 is located outside the building 105. In other embodiments, the heat exchanger 365 is located within the building 105 and remote from the shopping area 110.

The supply main 370 is coupled to the heat exchanger 365 to distribute coolant fluid from the heat exchanger 365 to an inlet branch line 395 that couples the supply main 370 to the refrigeration unit 325. The inlet branch line 395 is coupled to and in communication with the heat exchanger 345. An outlet branch line 400 couples the heat exchanger 345 to the distribution main 375 to deliver heated coolant fluid to the distribution main 375.

The distribution main 375 is coupled to the pumps 380 to distribute heated coolant fluid from the outlet branch lines 400 to the pumps 380. The illustrated pumps 380 are coupled to the distribution main 375 and the discharge main 385 in parallel with each other to pump coolant fluid from the distribution main 375 into the discharge main 385. In other embodiments, the pumps 380 can be connected in series with each other.

In operation, CO₂ refrigerant in the refrigeration unit 325 employing a transcritical refrigeration cycle is heated in the evaporator assembly 320 as it removes heat from the refrigerated area 330 of each refrigerated display case 315. The compressor assembly 340 compresses the heated refrigerant and forces it to flow to the fluid-cooled heat exchanger 345. The fluid-cooled heat exchanger 345 transfers heat from the CO₂ refrigerant fluid to the coolant fluid that flows through the heat exchanger 345 from the inlet branch line 395 to the outlet branch line 400. As a result of the heat transfer, the CO₂ refrigerant cools to a lower temperature and returns to the evaporator assembly 320 to be heated and cooled in a cyclical manner. In embodiments of the refrigeration unit 325 that includes the expansion device, the CO₂ refrigerant can be expanded to a lower pressure fluid prior to entering the evaporator assembly 320.

Coolant fluid in the coolant loop 317 flows from the inlet branch line 395 to the outlet branch line 400 in heat exchange relationship with the CO₂ refrigerant in the heat exchanger 345. The heat transfer between the heat exchanger 345 and the coolant loop 317 increases the temperature of the coolant fluid flowing through the coolant loop 317. The heated coolant fluid flows through the outlet branch lines 400 and collects in the distribution main 375. The pumps 380 pump the heated coolant fluid from the distribution main 375 to the discharge main 385. The pumps 380 generally drive coolant fluid flow throughout the closed coolant loop 317. The heat exchanger 365 receives the heated coolant fluid and discharges heat from the fluid to the environment. The cooled coolant fluid returns to the heat exchanger 345 via the supply main 370 and the inlet branch lines 395 to cool the refrigeration units 325 in a cyclical manner.

Various features and advantages of the invention are set forth in the following claims. 

1. A refrigeration system comprising: a plurality of refrigerated display cases, each of the plurality of refrigerated display cases including a dedicated evaporator assembly adapted to cool return air from the respective refrigerated display case by at least partially evaporating a refrigerant, and a dedicated first stage compressor assembly in fluid communication with the dedicated evaporator assembly to compress the refrigerant from the dedicated evaporator assembly to a first pressure corresponding to a first temperature of the refrigerant, and to discharge the compressed refrigerant into a discharge main; a second stage compressor assembly in fluid communication with the dedicated first stage compressor assembly of each of the plurality of refrigerated display cases to receive the refrigerant and to compress the refrigerant to a second pressure that is higher than the first pressure, the second pressure corresponding to a second temperature of the refrigerant; a heat exchanger located remotely from the plurality of refrigerated display cases and in communication with the refrigerated display cases via a fluid main, the heat exchanger including an inlet in fluid communication with the second stage compressor assembly to receive the refrigerant from the second stage compressor assembly to reject heat from the refrigerant to an environment, the heat exchanger further including an outlet in fluid communication with the dedicated evaporator assembly in each of the plurality of refrigerated display cases via the fluid main.
 2. The refrigeration system of claim 1, wherein the refrigerant includes a carbon dioxide refrigerant.
 3. The refrigeration system of claim 1, further comprising a vessel positioned in fluid communication with the fluid main between the refrigerated display cases and the heat exchanger to separate liquid and gaseous phases of the refrigerant and to supply liquid refrigerant to each dedicated evaporator assembly.
 4. The refrigeration system of claim 3, wherein the discharge main is in heat exchange relationship with the refrigerant in the vessel, and wherein the refrigerant in the discharge main is configured to be de-superheated by heat exchange with the refrigerant in the vessel.
 5. The refrigerated system of claim 4, further comprising a fluid line in fluid communication with the discharge main between the second stage compressor assembly and the vessel to direct the de-superheated refrigerant from the discharge main toward the second stage compressor assembly.
 6. The refrigerated system of claim 5, further comprising a bypass main coupled between the vessel and the fluid line to circulate gaseous refrigerant from the vessel toward the second stage compressor assembly without passing through the fluid main to the refrigerated display cases.
 7. The refrigerated system of claim 5, further comprising an outlet line extending between the second stage compressor assembly and the heat exchanger, and a bypass line coupled between the fluid line and the outlet line to circulate gaseous refrigerant from the fluid line to the heat exchanger without passing through the second stage compressor assembly when a temperature of the refrigerant in the discharge main is below a predetermined temperature.
 8. The refrigerated system of claim 7, further comprising a check valve positioned in the bypass line and configured to inhibit flow of the refrigerant from the outlet line backward to the fluid line.
 9. The refrigerated system of claim 1, wherein the refrigerant includes a critical temperature, and wherein the first temperature is a subcritical temperature and the second temperature includes a transcritical temperature.
 10. The refrigeration system of claim 1, wherein the dedicated evaporator assemblies are connected to the liquid main in parallel, and wherein the dedicated compressor assemblies are connected to the discharge main in parallel.
 11. A refrigeration system comprising: a low temperature refrigeration circuit configured to circulate a first refrigerant, the low temperature refrigeration circuit including a plurality of refrigerated display cases, each of the plurality of refrigerated display cases having a dedicated evaporator assembly adapted to cool return air from the respective refrigerated display case by at least partially evaporating the first refrigerant, and a dedicated first stage compressor assembly in fluid communication with the dedicated evaporator assembly to compress the first refrigerant from the dedicated evaporator assembly to a first pressure corresponding to a first temperature of the first refrigerant; a high temperature refrigeration circuit configured to circulate a second refrigerant, the high temperature refrigeration circuit including a second compressor assembly operable to compress the second refrigerant to a second pressure corresponding to a second temperature of the second refrigerant, and a heat exchanger configured to cool the second refrigerant; and a heat exchanger in communication with the low temperature refrigeration circuit and the high temperature refrigeration circuit to transfer heat from the first refrigerant to the second refrigerant to cool the first refrigerant without mixing the first refrigerant and the second refrigerant.
 12. The refrigeration system of claim 11, wherein the first refrigerant and the second refrigerant include carbon dioxide refrigerant.
 13. The refrigeration system of claim 11, wherein the first refrigerant is circulated in a subcritical refrigeration cycle.
 14. The refrigeration system of claim 11, wherein the low temperature refrigeration circuit further includes a low temperature receiver disposed between and in fluid communication with each dedicated evaporator assembly and the heat exchanger to separate liquid and gaseous phases of the first refrigerant and to supply liquid refrigerant to each dedicated evaporator assembly.
 15. The refrigeration system of claim 11, wherein high temperature refrigeration circuit further includes a high temperature receiver disposed between and in fluid communication with the heat exchanger and the heat exchanger to receive the cooled second refrigerant and to direct the cooled second refrigerant to the heat exchanger.
 16. The refrigeration system of claim 11, wherein the high temperature refrigeration circuit further includes an accumulator disposed between and in fluid communication with the second compressor assembly and the heat exchanger to separate liquid and gaseous phases of the second refrigerant and to supply gaseous phase second refrigerant to the second compressor assembly.
 17. The refrigeration system of claim 11, wherein the heat exchanger is operable as a condenser for the first refrigerant, and wherein the heat exchanger is operable as an evaporator for the second refrigerant.
 18. The refrigeration system of claim 11, wherein the second refrigerant is circulated in a transcritical refrigeration cycle.
 19. A refrigeration system comprising: a plurality of refrigerated display cases, each of the plurality of refrigerated display cases including a dedicated evaporator assembly adapted to cool return air from the respective refrigerated display case by at least partially evaporating a refrigerant; at least one first stage compressor assembly in fluid communication with the dedicated evaporator assemblies to compress the refrigerant from the dedicated evaporator assemblies to a first pressure corresponding to a first temperature of the refrigerant; a second stage compressor assembly in fluid communication with the at least one first stage compressor assembly to receive the compressed refrigerant and to compress the refrigerant to a second pressure that is higher than the first pressure, the second pressure corresponding to a second temperature of the refrigerant; a heat exchanger located remotely from the plurality of refrigerated display cases, the heat exchanger including an inlet in fluid communication with the second stage compressor assembly to receive the refrigerant from the second stage compressor assembly and to reject heat from the refrigerant to an environment; a vessel positioned between and in fluid communication with the dedicated evaporator assemblies and the heat exchanger to separate liquid and gaseous phases of the refrigerant; a liquid main fluidly coupled to the dedicated evaporator assemblies and to the vessel, the liquid main configured to supply liquid refrigerant to each dedicated evaporator assembly; and a gas main fluidly coupled to the second stage compressor assembly and to the vessel, the gas main configured to direct gaseous refrigerant from the vessel to the second stage compressor assembly without passing through the dedicated evaporator assemblies.
 20. The refrigeration system of claim 19, wherein the refrigerant includes a carbon dioxide refrigerant.
 21. The refrigerated system of claim 19, wherein the refrigerant includes a critical temperature, and wherein the first temperature is a subcritical temperature and the second temperature includes a transcritical temperature.
 22. A refrigeration system comprising: a closed coolant loop configured to circulate a coolant fluid, the coolant loop including a supply main, a distribution main, a discharge main, a heat exchanger fluidly connected to the discharge main and the supply main and in communication with an environment to reject heat from the coolant fluid into the environment, and at least one coolant pump fluidly connected to the distribution main and the discharge main to pump the coolant fluid to the heat exchanger and through the coolant loop; and a plurality of refrigerated display cases coupled to the coolant loop, each of the plurality of refrigerated display cases including a dedicated evaporator assembly adapted to cool return air from the respective refrigerated display case by at least partially evaporating a refrigerant, and a dedicated refrigeration unit coupled to the evaporator assembly, the evaporator assembly and the refrigeration unit in fluid communication with each other and defining a closed transcritical refrigeration circuit, the refrigeration unit including a compressor assembly in fluid communication with the evaporator assembly to compress the refrigerant from the evaporator assembly, and a heat exchanger in fluid communication with the compressor assembly and with the evaporator assembly, the heat exchanger in heat exchange relationship with the coolant loop to reject heat from the refrigerant to the coolant fluid. 